This invention relates generally to the treatment of aluminum containers and more specifically to a process for cleaning oil and debris from the surfaces of such containers so that a conversion coating can be applied.
Aluminum containers used in the food and beverage industry are provided with a conversion coating of, for example, phosphorous, chromium, zirconium or titanium salts which prevents the staining of the aluminum during the pasturization of beer or other food stuffs. The coating also aids in the adhesion of organic coating materials to the containers. A satisfactory conversion coating process requires that any dirt and lubricants be removed from the surface of the aluminum. In the container forming process known as drawing and ironing, fine particles of aluminum and other debris are pressed into the surface when the aluminum is formed into a can body. These particles and debris must also be removed from the surface. In the past, alkaline cleaners and etchants were tried and abandoned due to scale formation in the equipment, fluctuations in etch rate giving poor appearance and even leaks, and chemical contamination due to high levels of dissolved aluminum. Consequently, acid etching and cleaning are usually employed. The acid etchants include hydrofluoric acid. Although giving good results and producing the mirror-bright can surfaces which are preferred by the beverage manufacturers, the use of hydrofluoric acid presents a safety and effluent disposal problem and requires stainless steel equipment. An alkaline cleaning process for aluminum containers is disclosed in U.S. Pat. No. 4,091,954 which process employes 3 to 5% tetrasodium pyrophosphate, 0.1 to 0.2% sodium gluconate, and 0.1 to 2% of wetting agent. However, cleaning temperatures of 160.degree.-180.degree. F. are disclosed and the presence of wetting agent in the composition would be expected to create foaming which is undesirable.
We have discovered a low-temperature alkaline cleaning and etching process for aluminum containers using alkali metal hydroxides which cuts cleaning costs, alleviates the safety and effluent problems and reduces the amount of rinse water needed. At the same time, the process avoids the problems associated with previous alkaline etch processes. Bright can surfaces are produced which can be conversion coated without further processing. This is surprising in view of the fact that other aluminum cleaning processes, such as the immersion cleaning of lithographic plates, using similar alkali metal hydroxide etch solutions used in other aluminum cleaning operations require a subsequent desmutting step.